1. Field of the Invention
The invention relates to a control system for a continuously variable transmission, and more particularly, to a control system for a continuously variable transmission of a type having a plurality of power transmission paths selectively used according to vehicle running conditions.
2. Description of the Related Art
There have been known various continuously variable transmissions such as belt type continuously variable transmissions and toroidal type continuously variable transmissions for automobile vehicles which continuously vary a gear ratio between an input speed from an engine and an output speed to drive wheels to attain an engine speed determined according to vehicle running conditions including a vehicle speed and throttle opening. Such a toroidal type continuously variable transmission has a toroidal continuously variable gear mechanism which is comprised of input and output toroidal disks and rollers interposed between these input and output toroidal disks. The toroidal type of continuously variable transmission continuously varies the gear ratio according to inclinations of the roller of the toroidal continuously variable gear mechanism. One of the toroidal type continuously variable transmissions is known from, for example, Japanese Unexamined Patent Publications Nos. 3-223555 and 6-101754. It has been known that what is called a geared neutral starting system is employed in the continuously variable transmission regardless of belt type or toroidal type. In such a continuously variable transmission equipped with the geared neutral starting system, the toroidal continuously variable gear mechanism is mounted on a transmission input shaft connected to the engine, and a planetary gear set is mounted on a secondary shaft disposed in parallel to the input shaft. The planetary gear set is comprised of three rotary elements, namely a sun gear, an internal gear and a pinion carrier supporting pinions meshed with the sun gear and the internal gear. One of these rotary elements, i.e. the internal gear, is used as an transmission output gear. Rotation of engine is imparted to the planetary gear set partly directly through the pinion carrier and partly through the sun gear via the toroidal continuously variable gear mechanism.
The ratio of rotation between the pinion carrier and the sun gear is varied by controlling the gear ratio of the toroidal continuously variable gear mechanism so as to hold the transmission output element, i.e. the sin gear, remaining stand still, providing a neutral condition. By increasingly or decreasingly varying the gear ratio of the toroidal continuously variable gear mechanism causes the internal gear as the transmission output element to rotate in a forward direction or in a reverse direction. This type of continuously variable transmission permits the vehicle to start forward or back without use of a clutch and/or torque converter, which yields improvement of responsibility and power transmission efficiency of the continuously variable transmission.
The toroidal continuously variable gear mechanism is controlled to vary its gear ratio so as to gain a resultant transmission gear ratio or to provide a target engine speed which is determined on a control map specifying gear ratio change patterns according to vehicle running conditions including vehicle speeds and throttle positions. As is known in the art the gear ratio of the toroidal continuously variable mechanism is varied by controlling angles of inclination of the roller with respect to the toroidal surfaces of the input and output disks. However, since gear ratios and inclinations of the rollers of the toroidal continuously variable mechanism are fixedly correlated to each other, if inclining the roller all the way in one direction to attain an aimed desired gear ratio, the roller must be put at great angles of inclination with respect to the input and output disks to attain some aimed gear ratios, which causes a decline in power transmission efficiency of the continuously variable transmission.
It has been known to provide the continuously variable transmission in which power transmission paths for transmitting engine output power to drive wheels are changed from one to another according to vehicle running conditions to reverse the correlation between gear ratios and inclinations of the rollers of the toroidal continuously variable mechanism. The continuously variable transmission of this kind remains inclinations of the roller within specified limits of angle for all desired gear ratios. In order for the continuously variable transmission to establish a geared neutral condition, two power transmission paths are organized such that a first power transmission path transmits engine output power to the drive wheels via a toroidal continuously variable mechanism and a planetary gear mechanism to reduce the resultant gear ratio as the toroidal continuously variable mechanism increases its gear ratio in forward speed ranges and a second power transmission path transmits engine output power to the drive wheels via the toroidal continuously variable mechanism only to reduce its gear ratio as the toroidal continuously variable mechanism reduces its gear ratio. Considering a case of reducing the gear ratio of the continuously variable transmission following an increase in vehicle speed by way of example, the roller is inclined in a direction in which the toroidal continuously variable mechanism increases its gear ratio (this control of gear ratio is referred to as a low mode gear ratio control) after putting the continuously variable transmission in the first transmission path from the geared neutral position and subsequently in an opposite direction in which the toroidal continuously variable mechanism reduces its gear ratio (this control of gear ratio is referred to as a high mode gear ratio control) after putting the continuously variable transmission in the second transmission path. Accordingly, the toroidal continuously variable mechanism maintains inclinations of the roller within the specified limits of angle, preventing the continuously variable transmission from causing a drop in power transmission efficiency.
If changing the continuously variable transmission between the first and second power transmission paths when there is a difference in resultant gear ratio between the low gear ratio control mode in which the first power transmission is used and a high gear ratio control mode in which the second power transmission path is used, a steep change in gear ratio of the toroidal continuously variable mechanism as great as the passengers feel a shift shock occurs. For this reason, changing the continuously variable transmission between the low and high gear ratio control modes must be timely made at a point where the resultant gear ratio continuously varies between the low and high gear ratio control modes, i.e. the resultant gear ratios of the continuously variable transmission in the low and high gear ratio control modes are identical with each other. Accordingly, this point is a characteristic point determined based on vehicle speeds and throttle openings and lies on a gear ratio control line set forth for each specific throttle opening. When connecting the points on the gear ratio control lines set forth for specific throttle openings by a line, this line forms a mode switch line, an inclination angle of the line indicating the same resultant gear ratio described above.
When the vehicle changes its speed crossing the mode switch line, the continuously variable transmission changes the gear ratio control modes or the power transmission paths from one to another. When the vehicle enters a slope repeatedly sloping up and down with an accelerator pedal remaining fixedly stepped on, i.e. with an throttle remaining open to a fixed position, the vehicle changes its speed according to the up and down slopes. If the vehicle speed changes repeatedly crossing the switch point, the continuously variable transmission switches the power transmission paths frequently, which always cause hunting of a switch of power transmission paths. Further, since the driver does not changes an accelerator pedal position, the driver feels shocks accompanying switches of the power transmission paths. This problem is common to continuously variable transmissions which selectively uses a plurality of power transmission paths.